I never heard of a knock sensor; but usually knock refers to when a lower grade gasoline is put into a vehicle than that which it is designed to run on; and as a result the fuel detonates (spark plug fires) before it is supposed to which results in a knocking sound. You can hear a car knocking when this happens; but the the 2007 328xi BMW's are designed to run on a minimum octane of 87 (regular grade gas), 89 (mid grade gas) or 91 (hi test gas, which is designed to run on). I think you get less performance if you put octane 87 or 89 in the vehicle but it should run fine.